Serializing/Deserializing a bitmap to a specific pixelformat

I am serialising a bitmap which has pixelformat of 32bppArgb to file. On deserialising the pixelformat becomes 32bppRgb. The code used is based on http://social.msdn.microsoft.com/forums/en-US/asmxandxml/thread/e574e6fe-d889-4675-aa4c-d4002759821b

What has to be done to preserve the pixelformat?

Any suggestions?

Many thanks for any help